Holograms and diffraction gratings can be attached to a document or other articles containing printing and/or other information, such as a magnetic strip. For example, holograms are attached to credit cards in order to authenticate genuineness and increase the difficulty of counterfeiting credit cards. Holograms can be used as anti-counterfeiting devices on other types of documents, such as stock certificates, identification badges, passports, checks and even currency. Holograms and diffraction gratings are also attached to, or formed within, documents and articles for other reasons, such as for decorative effect. Packaging materials can have holograms and/or diffraction gratings for decorative or authenticating effects.
Holograms can be created by replicating the interference pattern of the hologram in a film via a surface contour in the film. When light is incident on this surface pattern, an observer views an image according to the information stored in the hologram. A diffraction grating can be made in a similar manner. When used for the purposes described above, the hologram or diffraction grating will typically be adjacent other materials, such as an adhesive, which can optically obscure the pattern of the device. There is a need therefore to preserve and enhance the patterns of the holograms and/or diffraction gratings.
More specifically, holographic and diffraction patterns for security and product enhancement purposes can be produced in a process which employs a carrier film onto which an embossable layer is deposited. The layers are embossed via a conventional holographic embossing process, after which the pattern is preserved and enhanced via vapor deposition of a metal, or vapor deposition of a material such as zinc sulfide which has an adequate difference of index of refraction versus the index of refraction of the embossable layer. In the vapor deposition process, the evaporated material forms a layer on the pattern, preserving the pattern. The pattern is further processed by either depositing a heat-activated adhesive or a pressure sensitive adhesive.
An example of the vapor deposition process for preserving a holographic pattern is a vacuum metallization process. In this process, a roll of material is placed in a vacuum chamber that contains a heating system and a roll of aluminum wire. The heating system includes an inter-metallic boat, furnace or crucible, that acts like a resistor and is subjected to very high heat. The aluminum wire is fed into the boat, and as it contacts the boat, the metal is vaporized. Concurrently, the roll of material containing holographic patterns is unwound, passed over a series of guide rolls and a chilled roll, and then rewound. The evaporated aluminum forms a layer on the film of material to preserve and enhance the image created by the holographic pattern. Although this known system has proven to be useful, the equipment required to perform the process is generally expensive to make and operate. Further, this method does not desirably lend itself to selective application of the metal, as is desired when the holographic pattern only covers a fraction the area of the material. Instead, the entire surface exposed in the chamber is covered by the vapor. Coating the non-holographic areas adds undesirable waste and cost to the process and end product.
What are needed in the art are preserved and enhanced holographic and diffraction patterns and a method for making them which are relatively inexpensive to make and operate, and which are capable of selectively applying the material to preserve and enhance the patterns.